This application relates to testers and, in particular, to testers for testing fused circuits, such as short circuit or grounded circuit detectors and indicators.
Many electrical systems include a plurality of fused circuits, the fuses of which are disposed in a central fuse panel or box. It is often convenient to test the circuits at the fuse panel, if it is at a relatively easily-accessible location. When testing circuits at the fuse panel, the tester is often applied to the circuit being tested, in parallel with the fuse. When testing for short circuits, however, this procedure may result in blowing the fuse.
It is known to provide circuit testers with a built-in circuit breaker, which can be connected to a fuse panel in place of a fuse for testing the fused circuit. One such device is sold by Snap-on Tools Company under the designation YA809, which is a short circuit locator. YA809 has a breaker with a single high-current rating and, when YA809 is connected to circuits having a lower current rating, this permits the flow of potentially damaging currents in the circuit being tested. Furthermore, YA809 requires the technician to be at the location of the fuse panel in order to view the provided visual indicator. This may be inconvenient, the technician may need to be elsewhere along the tested circuit while the testing is being conducted. Another diagnostic technique for locating a short or current flow path carrying a current excess is to repeatedly replace the blown fuse until the fault is located. This can waste a large number of fuses before testing is successfully completed. A more practical method of testing is to momentarily provide current flow by a resettable breaker and attempting to locate the short.
One tester model has a 30-A self-resetting thermal breaker that is installed across the blown fuse and repetitively allows current flow to the short circuit in the form of current pulses. The breaker opens after a short thermal delay and then automatically resets after cooling down. A magnetic field surrounding the shorted wiring is generated during the momentary high current pulses as a result of the repetitive breaker action. The tester includes a needle magnetic detector that deflects in response to magnetic field. By moving the magnetic detector along the wiring harness, the user is able to locate the short when the meter stops deflecting.
The tester can be difficult to use because the detector must be in close proximity to the wiring to work. Additionally, the repetition rate of the thermal breaker is on the order of tens of seconds, which causes the technician to wait a long time at a location for the thermal breaker to close in order to see if the short is in that location. Such waiting period creates difficulties when the location of short is difficult to reach. The longer duration of current pulse also causes exposure of the electrical system to a potentially damaging high level of current for a longer period during each pulse. Thermal breakers are prone to premature failure and instability in that their shut-off current depends on temperature, age and other external elements.
Another circuit tester, instead of allowing high current flow, uses high frequency AC signals transmitted into a short circuit and an associated receiver that is moved along the wiring. The tester works while power to the electrical system is either on or off. The short is located when the signal drops to zero. Thus the time taken to insure proper connection slows down locating the short. The cost of such systems is relatively high making it a less desirable tool.
Accordingly, there exists a need for a tester for electrical systems that reacts to a short circuit condition more quickly, locating electrical faults faster without subjecting the system to damaging high current flow for long periods. There is further need for a tester that can test electrical systems independent of the polarity of contacts in the circuit under test. There also exists a need for a tester that is operated by setting the breaker current limit in order to quickly find the level of current the circuit is drawing.